In the field of package production there is a demand for good printability of the paperboard. Good printing surfaces are normally created by clay-coating of the board surface. The board for packages is usually a multiply board with a bleached top layer under the clay coating. The term clay coating covers all types of coatings where pigments and binders are used together; However, clay-coated board is much more sensitive to water penetration during and after the sterilisation with steam than uncoated board. Therefore, clay-coated board is not at all suitable for production of packages that will be sterilized with steam.
The printing surface is the top layer of the board made of bleached cellulosic fibres. An improved top layer can be achieved if the board web is compressed in one or more roll nips, in most cases after the drying, in a calendering operation. An improved top layer can also be achieved by coating of the board with a coating colour, consisting of pigments and binders. In some applications of board for packaging of food it is a disadvantage to have a coating colour on the board surface, as the coating colour can reduce the hydrofobicity of the board. This is especially notable if steam is used for sterilisation of the board.
A recently developed extended soft nip calendering technique with substantially higher calendering temperature than normally used is disclosed in WO 01/29316. This technique can be used to provide a desired printing surface with low surface roughness, high gloss, and minimal gloss variation to uncoated paperboard, enabling the paperboard to be used in the production of printed packages. The roll temperature can be above 250° C. in the disclosed a extended soft nip calender. The top layer surface of the produced paperboard is resembles a coated rather than an uncoated product. In this context it can be mentioned that the extended soft nip calendering technique has previously been used to reduce the density of coated liquid board (WO 96/28609).
Further, there, is a newly developed process for packaging of wet food in packages made of laminated board, similar to the process of packaging food in tin cans. In this process the package and the food in it are sterilized together (U.S. Pat. No. 6,177,048). Steam is used for the sterilization, similarly to the production of canned food. If there are cut board edges on the package, steam and water will penetrate into the board through the cut edges. As the diffusion of water vapour cannot be prevented, steam will penetrate into the board through the edges and condensate. As the fibre surfaces become wet due to the condensation of the water vapour they will loose their hydrophobic character and water penetration due to capillary forces will occur. When this happens, the board gets soaked with water in a relatively short time.
In the field of packaging of liquid and/or wet food the board is often laminated with polyethylene or other plastic materials. The board must resist different sterilisation treatments, e.g. sterilisation with hydrogen peroxide. Edge-wicking (edge-soaking) at the cut and uncovered board edges is a particularly difficult problem. Resistance against edge-wicking is normally created by the use of different sizing agents, such as AKD (alkyl ketene dimer), rosin size and ASA (alkenyl succinic anhydride). The sizing agents can be used one alone or in combinations with each other, the combination of AKD and rosin size, known as dual sizing, being the generally accepted combination for liquid packaging board. The sizing agents are retained on the fibres during the paper making process and they spread over the fibre surfaces during the drying operation due to melting. Thereby the fibre surfaces will become hydrophobic and water penetration in the fibre structure of the board due to capillary forces is prevented. The general belief among scientists is that only a fraction of the fibre surfaces need to be covered in order to achieve a good hydrofobicity. Normally the AKD is based on stearic acid (C18) but palmitic acid (C16) or mixtures of C16 and C18 are also possible. However, according to accepted theory, the diffusion of water vapour into the board cannot be prevented by sizing.
It is well known [Roberts J. (1997): “A rewiew of advances in internal sizing”, Proc. The 11th Fundamental Research Symposium in Cambridge, pp 209-263] that maximum resistance against water penetration due to capillary forces is reached at relatively low addition levels of sizing agent, i.e. with 0.015% (0.15 kg/ton) reacted AKD in the sheet. All the added AKD is not retained in the sheet and all the retained AKD will probably not react. Experience has shown that addition levels of 2 kg of AKD per ton of dry fibres gives a sufficient margin in order to achieve the maximum hydrofobicity possible and necessary in the production of liquid packaging board. An addition of rosin size, between 1 and 1.5 kg/ton, is believed to improve the resistance against hydrogen peroxide, but it does not improve the hydrofobicity as such.
The pore structure is of great importance for water penetration into paper and board. The Washburn equation describes penetration of liquids into parallel capillaries, but it is also used to give an approximate description of water penetration into paper and board. In order to minimize water penetration according to the Washburn equation, the fibre surfaces should have as low surface energy as possible (i.e. maximum hydrofobicity) and the pore radii should be as small as possible. Small pores are created by relatively intensive beating of the fibres in refiners and/or by wet pressing of the wet board web with high pressure in the press section of the paper machine. When creating the small pores by beating and/or wet pressing, density of the board will increase. Therefore, high density of the paperboard reflects small pore sizes.
It would thus be desirable to be able to produce uncoated paperboard for packages with superior resistance against water penetration enabling water vapour sterilization of the package. As good printability is important for high quality packages of today, it would be desirable to be able to produce the board with a printing surface that is comparable to clay-coated board without compromising with other quality requirements of the product.